This background considers the variant α7 subunit as it relates to the nicotinic acetylcholine receptor (nAChR). The nAChR is comprised of transmembrane polypeptide subunits that form a cation-selective ion channel gated by acetylcholine (ACh) and other ligands. The hydrophobic transmembrane 2 (“TM-2”) region from each subunit is believed to form the wall of the ion channel.
Two of the more prominent nAChRs in brain are those containing α4 subunits and those containing α7 subunits (Sargent (1993) Annu. Rev. Neurosci. 16:403–443; Court et al. (1995) Alzheimer Disease and Associated Disorders 9:6–14). Mutations of the α4 and α7 subunits may underlie some diseases of the nervous system. For example, mutations of the α4 subunit have been associated with some forms of epilepsy (Beck et al. (1994) Neurobiol. Disease 1:95–99; Steinlein et al. (1995) Nature Genetics 11:201–203). Additionally, α7-containing nAChR may be involved in sensory processing related to schizophrenia (Freedman et al. (1995) Biol. Psych. 38:22–33; Rollins et al. (1995) Schizophr. Res. 15:183; Stevens et al. (1995) Psychopharmacol. 119:163–170), cytoprotection (Donnelly-Roberts et al. (1996) Brain Res. 719:36–44; Akaike et al. (1994) Brain Res. 644:181–187; Martin et al. (1994) Drug Dev. Res. 31:135–141; Quik et al. (1994) Brain Res. 655:161–167), and neurite growth and innervation (Chan et al. (1993) Neurosci. 56:441–451; Pugh et al. (1994) J. Neurosci. 14:889–896; Freeman (1977) Nature 269:218–222; Broide et al. (1995) Neurosci. 67:83–94).
A splice variant involving the TM-2 region of the α7 subunit has been detected in bovine chromaffin cells (García-Guzmán et al. (1995) Eur. J. Neurosci. 7:647–655), and a naturally-occurring mutation of a protein homologous to the α7 subunit found in Caenorhabditis elegans, leads to neurodegeneration (Treinin et al. (1995) Neuron 14:871–877). The latter is a single amino acid mutation in the TM-2 region similar to the chick α7 valine-251 to threonine (“c-α7V251T”) mutation, one of several mutations artificially introduced into the chick α7 subunit to facilitate the study of α7 nAChR structure and subunit function (Bertrand et al. (1995) Sem. Neurosci. 7:75–90).
Compared to the chick α7 wild-type (“c-α7WT”) nAChR, c-α7V251T (also referred to as α7–4) retained high calcium permeability but desensitized slowly, and was 180-fold more sensitive to ACh. In addition, the c-α7V251T nAChR responded to dihydro-β-erythroidine (“DHβE”), normally an nAChR antagonist at α7 and other wild-type nAChR, as if it were an agonist (Galzi et al. (1992) Nature 359:500–505; Bertrand et al. (1993) Proc. Natl. Acad. Sci. USA 90:6971–6975). These studies have led to a model delineating the structure of the pore-lining TM-2 region, and the hypothesis that specific mutations within the TM-2 region can generate ligand-gated ion channels that conduct current in the receptor-desensitized state in addition to the normal receptor-activated state (Bertrand et al. (1995), supra; Bertrand et al. (1992) Proc. Natl. Acad. Sci. USA 89:1261–1265; Galzi et al. (1995) Neuropharmacol. 34:563–582).
Although the chick α7 nAChR is pharmacologically similar to the mammalian α7 nAChR, there are significant differences. For example, 1,1-dimethyl-4-phenylpiperazinium. (“DMPP”) is a very weak partial agonist in the chick α7 nAChR, but is a highly efficacious agonist at the human α7 nAChR (Peng et al. (1994) Mol. Pharmacol. 45:546–554). Despite these differences, it would be expected that amino acid changes in the human α7 nAChR that are analogous to those in the chick α7 nAChR, particularly in critical TM-2 amino acids, would result in similar pharmacological and electrophysiological changes.